The static head, sometimes referred to as the pressure head, is a term primarily used in Hydraulics to denote the static pressure in a pipe, channel, or duct flow. It has the physical dimensions of length (hence the term "head") and represents the flow-work per unit weight of fluid. In practice the static head is equivalent to the vertical distance from a given streamtube to the piezometric head line or the hydraulic gradient line. If P is the pressure in a streamtube, the static head is defined by P/ρg. Using this definition it is apparent that in a open channel flow, where the pressure is assumed to be hydrostatic, the static head is the distance from the streamtube to the water surface.